Tuning system for resonant circuits



Jan. 16, 1951 H. BEDFORD ET AL TUNING SYSTEM FOR RESONANT CIRCUITSOriginal Filed Feb. 25, 1944 ,mfg-f- OSCILLATO :RTHUR HENRYASHFORD WYNNPatented Jan. 1.6, 1951 TUNING SYSTEM FOR RESONANT CIRCUITS LeslieHerbert Bedford, London, and Arthur Henry Ashford Wynn, Sheiield,England, assignors to A. C. Cossor Limited, London, England, a Britishcompany Continuation of application Serial No. 523,890, February. 25,1944.' This application April 13, 1949, Serial No. 87,2768.

ruary 26, 1943 6 Claims.

Thisv invention relates to tuning systems for resonantk electriccircuits, and is particularly applicable to radio equipment intended foroperation on frequencies of the order of megacycles or tens ofmegacycles. Y q

One feature of the invention is a resonant circuit Which is capable ofhaving its tuning varied, continuously or in steps, over a range, andwhich is so arranged that at any setting of its tuning it is possible,by a simple switching operation, to change the reso-nant frequency by apredetermined smallY amount independent of the setting. Another featureofthe invention is a resonant circuit wherein the rate of change 0fresonant frequency, over a small range, with variations of one ofthevariable capacitances comprised therein, is independent of the absolutevalue of the resonant frequency when this is varied by changes of othercapacitances in the circuit.'

Another feature of the invention is concerned with means for easily andaccurately tuning oscillators. According to this feature, an oscillatoris provided with a step-by-step coarse tuning control and a fine tuningcontrol, and also with a trimming control for adjusting the resonantfrequency at any setting of they coarse tuning control. While suchadjustment is carriedv out, the fine tuning control is set eitherat zeroor at Some other predetermined setting independent of the setting of thecoarse tuning control. The fine tuning control is so designed that itsown calibration is substantially correct for all settings of the coarsetuning control.

. Another feature of the inventionr is the provision of simple means forcomparing the resonant frequency of such an oscillator with a standardfrequency at every setting of the coarse tuning control, sothat thetrimming control can be adjusted correctly for that setting.

This application is a continuation of our abandoned earlier ledapplication, Serial No. 523,890, filed February 25, 1944, and certainsubject matter of the earlier filed application is also being claimed inapplication, Serial No. 75,412, filed February 9, 1949.

In the accompanying drawings, Figures 1 and 2 are circuit diagrams ofvariably .tunable resonant circuits with alternative arrangements' forchanging the resonant frequencyfby the opera- @9119i switch by a .xedammi independent (Cl. Z50-40) In Great Britain Febof the tuning of thecircuit; Figure 3 is a circuit diagram of a variably tunable resonantcircuit having coarse and ne tuning controls, and in which vthecalibration of the fine tuning control is substantially independent ofthe setting of the coarse tuning control; and Figure 4 is a schematicdiagram of a superheterodyne radio re-r ceiver, the local oscillator oflwhich has a tank circuit corresponding to Figure 3.

vCircuits such as are shown in Figures 1 and 2 are required in highfrequency radio equipment which serves alternately as a transmitter andas a superheterodyne receiver, both operating on thesame signalfrequency, and wherein the same oscillator is used for sending andreceiving. In such an equipment, the resonant Vfrequency must bechanged, by an amount equal to the relatively small intermediatefrequency, when switching over from sending to receiving. In each of thecircuits shown, this result is achieved by switching an additionalcondenser into or out of circuit.

In the arrangement of Figure 1 an inductance L is tuned by the resultantcapacity of a network of condensers I, 2,3 and 4, the condenser 4 beingconnected in parallel with the condenser 3 through a switch S which maybe opened or closed to provide a predetermined change in the resonantfrequency of the circuit. The condensers I and 2 are variable and gangedtogether, and in order to ensure that the frequency change brought aboutby actuation of the switch S is constant and independent of the settingof the two ganged condensers l and 2, it is arranged that the lawsrelating change of capacitance with change in setting of the gangingmeans is different for the two condensers l and 2.

The inter-relationship of these two laws may conveniently be expressedby the equation T* Ogg.. gzlaaica where C2 is the capacity of condenser2,

C3 is the capacity of condenser 3,

C4 is the capacity of condenser 4,

f is the resonant frequency, that is to say 1 27m/C-z Where CX is theresultant capacity of condensers I, 2 and 3 (wref-fes) L is the value ofinductance L, and F is the iixed frequency change. effected by closingswitch S.

The Vane shapes of condenser 2 will, ofcourse, depend upon the vaneshapes of `conc'enser If condenser I has a straight-line-capacity lawand the maximum values ofcondensers-I and 2 are fairly nearly equal, theyaneshapes-ffor condenser2 are particularly convernent. Y A

In practice small trimming condensers will usually be provided inparallel with condensers 2 and 3 for final adjustments of the circuitsso that the tracking is as nearly accurate as possible.

'the alternative arrangement sho'vvn -in-Fig'- ure '2, switch S connectsa Xed 'condenser '5 in parallel With condenser 2. -1This arrangement hasan advantage over that shown iinlFigure l in that theV switch can-befcon'n'eetedtofone side of each of thefvariable condensers I and 2, andprac'- tice niayftherefoie-be earthed. AThe vane shapes, hvvever,Valthough practicablebecome more avvk- Ward than those obtained yvithFigure l, and it isprefe'rredto v'arrange that condenser l has astraight-line-c'apa'city l'avv'and that the maximumvalues of, condensers-i and 2 v'are fairly equal.` The law-'relating the ycapacity ofcondenser -2 Vwith frequenfcy this example takes the Yforr'n:

tir-here G is the capacitance of condenser's.

e ycapa'cities of the vcnderisers "il and 5 in the arrangements efFigures land -2 respectively may be f the sameforder as that 'ofcondenser 3 and may assist in sWam-pingstray capacities.

"1hef`1'circuit ``shown in Figure 3 is vgenerally similar in principleto that of Figure 2J `but the tuning is--variable over a range b'y aiinetuning-control having lXed calibration, instead of being merelycapable cf `alixed'change by a switching operation. 'HIhei-lxedcondenser which is switched linte and out of circuit in the arrangementsof Figures l and 2 becomes, for thisl purpose, a vari'- able condenser,preferably of -straight-line-capacity lavv.

`In thejcircuit illustrated in Figure 3, theco'a'rse tuning ofinductance L is eected 4by variable condenserI, which is one ofthejsection's ,of 'a .gange'd condenser controlled by thejcoarse tuningcontrol knob. This control is preferably provided with a step-by-stepmechanism whereby approximately equal frequency steps of, say, 0.1megacycle per second each may be eiected.

The ne 'tuning' tof the lcircuit is effected by variable condenser 5.Itqwill normally be convenient that the fullrange of frequency changeproduced by condenser 5 shall be equal to the frequency change producedby moving the ccn= trol of condenser 4 I through one step. Theccn'e trolof condenser 5 may also be provided with a step-by-step mechanism,vwhereby for each step the change of frequency is, say, 0.01 mc./s.

,y of condenser 1I, andthis is achieved by connecting vc 'nden"seracross condenser 2 in the same In order to achieve that the rate ofchange of frequency With change of capacity of condenser 5 shall beindependent of the setting of condenser I, condenser 5 is connectedacross one half of the chain of condensers 2 and 3, which are connectedin series across condenser I and one of which, condenser 2, is gangedWith condenser I. The required result is then approximately achieved ifthe capacity, of condenser 2 Varies withath'at of condenser I in`accordane'with a special law;A V Condenser 6 is provided in parallelwithv the fine tuning condenser 5 for the purpose of trimmingtl'letuniirg .te correct for errors in the calibration .of condenser-4I. It is convenientfthat trimming 'condenser' B should have a range offrequency crrectn of the order of plus or minus yhalf thefiequencyhangecorresponding to one step of condenser I. This range of correctionyshculdbe Substantially independent of the setting mannei' as condenser5 is connected. An incidental-advantage of connecting the fine tuningcondenser 5 and the trimming condenser 5 in parallel with one half "ofthecha-inicf condensers 2 Aandf 3 is that .they-are not. then' requiredto 'be' so unreasonably-small as they would be fifcon'- nected directlyin parallel 'with 'condenser-Il v Means should be providedfor check-ingthe calibration ofthe control of. condenser :I atfevery step initsfrange This .may be done'byeornparison Withstandard `frequenciesWhich-will usually cor-responcLat all steps of condenser I, either tothe zero setting of .the iineftuning control ,5,or else,to-somefotherfsingle settingl thereof-:such as :itsmid-setting. ft; isdesirable-to ebviate the necessity of. always turn-ing thelne `:tuningcontrol to this setting before nalibratmg: and to this end pre-setcondenserv is providedytcgether Wit-h a switch 'I'Whereby it ymaybesubstituted for-condenser 5. jSWitch1-'I'- may be :gangedwith -a switchwhich bring-s.` into la'ctionthe system :for: comparison with thestandard.- .-1 These .switches mayrbe operated rby depressingaiknobLwhichthenbecomes keiectiva-` by sul)sequentrrotation",to'adjnstcondenser- 6. Gondelnserisfpreesetto the -value corresponding tothecapacity of cicndensergy .when atg-itsfzero--setting or otheryselectedsetting/,nue allowance :Y being made .for any; :change vin ,-straycapacities resulting from-the: changer-over. f They greater thefratio of1 maximum capacity .fof condenser 2 to'maximumcapacity nfscondenser IIis made.,v the nearer is it pessiblete apprcaclq--the ideal thatA the:true l calibration-bf lcondenser-i5 in frequency increment Ishould :beuindependentfof' VIn one specific;example,A` Where thecircuit is tunedIoverrthe range4-45 110272415 mcs/sl, the coni-` Fixed condenser 3 ..Y Al `15'() Variablefcondenser ..v 0-50 Trimmer "condenser 6 0540caffetteria-ing capa-eines ci cfiqenjser ,ir 'afreiuuie straw-ana@intenser-l2 (including-stays' acrossitself and across condensers 5 and6) lie on a smooth curve through the following points;

Capacity of condenser r.f.) Capacity of condenser The superheterodyneradio receiver illustrated by Figure 4 has a receiving aeriall 9, aradio frequency ampliiier l0, a local oscillator I2, a mixer I3, anintermediate frequency amplifier I4, a detector I5, an audio amplifier22 and a sound reproducing device shown as headphones 23, all inconventional arrangement. It is also provided, in conventional manner,with a beat oscillator capable of generating oscillations at and aboutthe intermediate frequency, for the receptionof C. W. telegraphy, andwith a switch I9 for bringing this into or out of circuit according towhethertelegraphy or telephony is to be received.

`In addition, this receiver is provided with a crystal harmonicgenerator II and with ganged switches 7, I8, 20.

The local oscillator I2 has a tank circuit of the form i.lustrated inFigure 3 and switch 'I Ycorresponds with switch 'I' of Figure 3. Tuningcondensers in radio frequency amplifier IIl will be ganged withcondensers I and 2 and with condenser 5.

The ganged switches 1, I8 and 2B are shown in the attitude for radioreception. When they are thrown to the alternative attitude, switch "isubstitutes pre-set condenser S for condenser 5, switch I8 disconnectsthe signal input circuit comprising radio frequency amplifier I from themixer I3 and substitutes crystal harmonic gen-V` erator II, and switch2i) connects the beat oscillator I6 to the input of detector I5 in thesame manner as does switch I9. In this alternative attitude, thecircuits are connected for checking and correcting the coarse control ofthe local oscillator frequency, whatever its setting and independentlyof the setting of the fine tuning control.

It will 'be assumed by way of example that the radio receiver isrequired to operate over a radio frequency range from 4 to 7 mc./s., andthat the coarse tuning control has a step-by-step mechanism whereby itmay be set for the reception of any frequency within this range which isan integral multiple of 0.1 mc./s.y The intermediate' frequency shouldbe a low odd multiple of half this frequency step, i. e. "a low oddmultiple of 50 kc./s., say, 450 kc./s. The crystal harmonic generator II will be required to produce a substantial output at all frequenciesnear the range of tuning of the local oscillator, which are evenmultiples of said frequency step, i. e. at every frequency near therange 4 to 7 mc./s., which is a Aharmonic of 200 kc./s. When the gangedswitches l, I8, 2t are thrown to the attitude for calibration, and theoscillation from local oscillator I2 is mixed with the output fromcrystal harmonic generator I I, the intermediate frequency amplifier I4passes only those difference beat frequencies which are near to 450kc./s. It will be found that, for any two adjacent settings of thecoarse control of the local oscillator, one will beat with a highercrystal harmonic and the other with a low crystal harmonic to produce adifference beat frequency which, when the trimming is correct, will beequal to the intermediate frequency. This will be seen from thefollowing table. The rst column shows the calibrations of a series ofadjacent settings of the coarse tuning control. This is,v of course,-l

calibrated in terms of the radio frequencytol be received. The secondcolumn shows the actual local oscillator frequencies corresponding tothese :i

settings. The third column shows the crystal harmonic frequencies, allbeing'harmonics of 200 kc./s., which will beat with the localoscillatory,

frequencies to produce difference beat frequencies of 450 kc./s.

Radio Sig- Local Oscil- Crystal nal frelator fre- Harmonic quency,quency, Irenuency,

mc./s. mc./s. mc./s.

In this example, it is assumed that the local' oscillator frequenciesare on the lower side of the radio frequencies; but the same result willbe obtained if they are on the higher side.

In the detector I5, the output of intermediate.l

frequency amplifier I4 is mixed with that of beat oscillator I6.frequency variable over a-range of a few kcJs. for the selection of apreferred pitch for C. W. telegraphy reception. Its frequency controlmay have a click positionA for approximately 450v kc./s., in whichposition it will be placed when Calibrating. The probable error in thissetting is-v so small relative to the radio frequency that no greataccuracy of setting or stability in the 4best oscillator is required.The difference beat fre-y quency between the output of beat oscillatorI6 and the output of intermediate frequency ampli-- fier I4 willrepresent the error in tuning of the local oscillator. This beatfrequency can be heard in the headphones 273, and the trimming condenser6 can be adjusted until the beat fredispensed with, because the beatoscillator I6 is not employed in the Calibrating process. 'Theintermediate frequency is preferably again a low oddfmultiple of halfthe frequency'step, again say 450 kc./s.

When the frequency of the local oscillator I2 is` near to an oddmultiple of kc./s., two of the difference beat frequency components willpass the intermediate frequency amplifier. One of these will be thedifference beat between the oscillation to be checked and a crystalharmonic of high frequency, and the other between said os-v cillationand a crystal harmonic of lower frequency. For example, if the frequencyof the local oscillator I2 is 6053 kc./s. and the pass band of theintermediate frequency amplifier 450i5 kc./s., then the beat frequenciespassing to the detector I5 will be 447 and 453 kc./s., producedrespectively by the beating of the local oscillator output against thecrystal harmonics of frequency 6500 and 5600. y

The two difference beat frequencies passing the intermediate frequencyamplier will themselves interact to produce a beat signal of differencefrequency which, in this example, will be 6 kc./s.,

This beat oscillator I6 willhave a.

assenso through and beat with the two diiference beat' frequencieanwhichwill be equally spaced from this intermediate frequency, one below andthe other above. The only diierence in effect will be that the finalnote will be halved in fundamental frequency. Y

In the superheterodyne radio receiver illustrated in Figure 4, it is notessential to use, as the tank circuit in the local oscillator l2, acircuit such as that shown in Figure 3. The required independenceVbetween the calibration of the iine tuning control and the setting ofthe coarse tuning' control can be achieved by using for the coarse'tuninga straight-line-frequency variableV condenserand by providing'mechanical gearing whereby the normally stationary member of thatcondenser ifs-rocked through a small angle by the ne tuning control.Resilient means'shouldbe provided "to permit' this normally stationarymember to be returned toits zero or other predeter-Y mined attitude,independently of the setting of the fine tuning control knob,- when thelcalibrating-system brought into operation. v

The superheterodyne receiver illustra-ted in Figure -4-Will serveverysatisfactorily as a wavemeter The signal, the frequency of which is tobe determined; isfappliedto the radio frequencyamplie'r I0; and thecoarse and'i'ne tuning controls areadjust'ed until the signalcomesthrough at maximum strength in the headphones 23. When therequiredsetting of the coarse tuning control has been-found,V theswitches 7, l'and 253 are temporarily 'thrown' over while the trimmingcondenser 6. is adjusted. These switches 'are then returned to thereceiving` attitude before final adjustment of the fineV tuning control.Thexfrequency of thev received signal can then beA read off from thecalibrationY of the coarse and ne tuning controls; If the signalunderexamin'ation is unrriodulated, the C, W. beat oscillator must bebroughtiintouse. A valve voltrneter may be sub-fY stitutedfor thedetector and headphones aga" means of detecting when the beat frequencyis re'- duced to zero.

We claim: l. A resonant circuit comprising an inductor,

a variable capacitor connected in parallelrwithz t said'inductorY andproducing equal changes incapacity forequal movements of its movableelement, a series arrangement of a Xed capacitor and a secondvariablecapacitor also connectedr in parallel with said inductor, meansproviding mechanical ganging between said variable ca' pacitors, andafourth capacitor connectedA intion insad'fourth'capacitorproducesaccnstant change in the resonance frequency of the circuitirrespective of the :setting of said'ganged condensers.- Y

2. Al resonant'circuit comprising an inductor of inductance L, avariable capacitorconnected injparallel withsaid` inductor andproducingequal changes in capacity forequal movements" of 'its movable element, aseries arrangement of a fixed`4 capacitor and asecond variablecapacitor' also connected in parallel with said inductor, 4meansproviding mechanical ganging between A said variable capacitors, asecond fixed capacitor and meansv to connect saidsecondxed capacitor inparallel with said .nrst iixedcapacitor, said seriesconnected variablecapacitor having a capacity* displacement.characteristic such thatits-capaci-V tance C2 .varies'throughoutits range of variationinaccordance with theV law:

Carcapacitance of the lrstfixed capacitor C4=capacitance of the secondiixed capacitor ,f1-.resonant frequency and F=nxed change of frequencyeffected by connect-i ing the secondxedcapacitor. Y

3. A resonant circuit'comprising an inductorof inductance L, a variablecapacitor'connected-in parallel with said inductor and producing equalchanges in capacity for equal movements ofits movableelement, aseriesarrangement of a nxed capcitor and a second variable capacitor alsoconnected in parallel with said inductor meansv providing mechanicalvariable capacitors, a second fixed capacitor and meansto connect saidsecond xed capacitor in parallel with said series-connected variablecai-- pac-iter, said seriesfconnectedvariable capacitor having. avcapacity-displacement characteristic suchtliat its capacvit'ance C2varies throughout its rangeA ofV variation-in accordance with the lav/:l

where C3=cap`acitance of 'seriesconnected fixed capacitorCsz'capacitance of secondlxed capacitorA relating change in*.capacitancethereofI with movement" of-l saidrganging means 'to produce a .gangingbetween saidV` substantially constant change in resonant frequency ofthe circuit for any given change in the setting of the finely-variablecapacitor irrespective of the setting of the coarsely-variablecapacitor.

5. A resonant circuit comprising an inductor, a Variable capacitorconnected in parallel with said inductor, a series arrangement of afixed capacitor and a second variable capacitor also connected inparallel with said inductor, means providing mechanical ganging betweensaid variable capacitors, a second fixed capacitor, and means to connectsaid second fixed capacitor in parallel with the first said xedcapacitor, said variable capacitors having diierent capacitydisplacementcharacteristics to produce different changes in capacity in response tomovement of said ganging means so that actuation of said means toconnect said second fixed capacitor in parallel with the rst said fixedcapacitor produces a constant change in the resonant frequency ofthecircuit irrespective of the setting of said gangng means.

6. A resonant circuit comprising an inductor, a

variable capacitor connected in parallel with said inductor, a seriesarrangement of a fixed capacitor and a second variable capacitor alsoconnected REFERENCES CITED The following references are of record in thefile of this patent:

Wireless World for November 17, 1938.

